As is known, in order demount a tire from a rim, it is first necessary to break the bead of the tire, i.e. separate its beads from the respective edge of the rim and then force, by means of a suitable tire mounting-demounting tool, at least one bead of the tire to go beyond the edge of the rim facing thereto. The bead breaking operation is achieved by means of a so-called bead breaker tool, which can be of fixed type or it can be configured as an idle roller.
The most recently conceived tire mounting-demounting machines comprise a wheel-holder table mounted arranged to rotate on a support base and a column rising from the base, from which one or two telescopic arms extend overhangingly; a bead breaker roller is supported at the free ends of such arms. Each bead breaker roller is normally frustoconical, rather flat, is axially and rotatably supported with tilted rotation axis, e.g. 45°, and contained in a plane passing through the longitudinal axis of the its respective telescopic arm, so that the roller has its distal (end) portion (that which in use is closer to the tire to be bead broken) further away from the column with respect to its own proximal (end) portion (with respect to the column).
In order to carry out a bead breaking operation, after having fixed a tired wheel on the wheel-holder table of the tire mounting-demounting machine, the or each bead breaker roller is brought, typically by means of controlled extension/retraction of the respective telescopic arm which supports it, above the wheel, such that the bead breaker roller is situated at the bead zone of the tire to be bead broken. The approaching of the roller to the side of the tire and to the flange of the wheel rim is then operated, until the distal end of the roller abuts against the flank of the tire and against the flange. At this point, the operator controls (e.g. by means of extension of the telescopic roller-holder arm) a movement of the bead breaker roller in a substantially radial direction (with respect to the rim) and its distal end is then forced to penetrate between the edge of the rim and the bead of the tire. Finally, the wheel-holder table is rotated in order to complete the bead breaking operation over the entire length of the tire bead.
With the bead breaker rollers proposed up to now, a constant intervention of the operator is therefore necessary, as such operator must first drive the advancing of the roller towards the axis of the wheel-holder table and then drive an advancing-lowering of the roller itself between the bead of the tire and the flange of the rim. Following an imprecise driving of the roller by the operator, it can occur that the tire or the rim is ruined, or that the advancing and advancing-lowering operations of the roller must be repeated several times before correctly completing the bead breaking.
In order to remedy this drawback, it was proposed to use so-called feeler pins mounted at each bead breaker roller. Each feeler pin is intended to detect, as the approaching is carried out of the respective bead breaker roller to a tire to be bead broken, the position of the roller with respect to the flange of the rim, and upon reaching correct position, sending a signal to a suitable controller which controls the extension-moving back and lowering-return movements of the respective telescopic arm, so as to obtain the penetration of the bead breaker roller between tire and rim in a nearly automatic manner. Such feeler pins and the controller nevertheless constitute a rather costly system and involve a considerable increase in the production and maintenance costs of the tire mounting-demounting machines on which they are mounted.